Thermal conductivity: λ (lambda)
A material’s thermal conductivity is its ability to transfer heat. The lower the thermal conductivity, that is, the smaller the value of λ, the greater the material’s insulating power. Thermal conductivity is measured in W/m.K (watts per metre Kelvin – 1 K is equivalent to 1° Celsius).
Thermal resistance: R
Thermal resistance, R, is another indicator of insulating power. It expresses a material’s ability to resist heat flow. The higher the R value, the greater the material’s insulating power. A material’s R value, measured in m2.K/W (metre square Kelvin per watt), is obtained by dividing the material’s thickness by its thermal conductivity.
NB: In practice, it is not enough to compare the R values of different materials because R values are stated for equal thicknesses of material. In fact, what the R value tells us is that you will needed greater thicknesses of some materials than others in order to achieve the same insulating power. For example, a 1-cm-thick layer of glass wool will provide the same amount of insulation as a 30-cm-thick brick wall.
Thermal transmittance: U
Thermal transmittance, U, is the rate of heat transfer through one square metre of a material. It is measured in W/m2.K.
Recommended R values to achieve a good level of insulation for roofs, walls and floors:
Roofs: 4.5 (minimum) to 8 (ideal)
Exterior walls: 2.5 to 4
Floor directly on the ground: 1.5 to 3
Floor above an unheated space: 3.5 to 6.
The thermal characteristics of materials
The materials used to build a house have different thermal characteristics (heat capacity, thermal diffusivity, thermal inertia) that have to be taken into account when calculating insulation requirements.
A material’s heat capacity is its ability to store heat. The materials used to build a house have very different capacities for storing heat energy. As a general rule, the denser the material, the greater its ability to conduct heat and the greater its heat capacity. Conversely, low-density materials tend to have lower heat capacities and therefore greater insulating power. Bioclimatic engineering makes use of these two contrasting properties in order to maximise the comfort of a building’s interior environment. Low-density materials are used to thermally insulate the building from its external environment, whereas high-density materials, inside the insulating envelope, facilitate the regulation of variations in temperature.
These high-density materials may simply form part of the masonry (concrete, stone, brick, etc.) used to build a home. As long as they are within the insulating envelope, they will give the property the thermal inertia it needs to smooth out variations in temperature over the year, thereby helping to keep the interior at a comfortable temperature.
A material’s thermal diffusivity is its ability to transfer heat to and from the surrounding environment. A dense material such as marble is able to store large amounts of heat energy. Because of its high thermal diffusivity, marble will usually absorb heat when it is touched and will therefore feel cold. In simple terms, although dense materials have high thermal inertia and can therefore store relatively large amounts of heat energy, they generally feel cold. Conversely, light materials such as cork have much lower heat capacities than marble. Because of its low thermal diffusivity, cork cannot absorb much heat so it generally feels warm to the touch. Even though light materials feel “warm”, they cannot give a building the thermal inertia it needs to ensure a comfortable environment.
Thermal characteristics of construction materials
In hot countries, it is extremely important to ensure buildings have a high level of thermal inertia, as this inertia will help regulate differences in temperature between day and night. Structural components with high thermal inertias will absorb the sun’s heat during the day and release this energy during the night. Thermal inertia combined with night-time ventilation is the most effective, economic and environmentally friendly way of ensuring building interiors remain cool during the hottest months of the year. And the high thermal diffusivity of the materials that give the building its thermal inertia will increase the feeling of comfort. In higher latitudes, the situation becomes more complex. Although high thermal inertia is still beneficial, the “cold-wall” effect associated with some materials is less agreeable during cooler times of the year. Inertia is especially important with non-controllable sources of heat (sun, wood stove without a thermostat). In northern Europe, where the sun is never very strong, a building with an effectively controlled heating system will not need high inertia. In addition, lining the interior of the building with “warm” materials will make it more comfortable.
Conversely, a building in southern Europe with south- and west-facing windows and heated by a wood stove will need a high level of thermal inertia. Of course, there are innumerable intermediary cases between these two extremes, which is why each project is unique.
Thermal bridges are responsible for between 5% and 25% of heat losses from a building.
Ideally, a building’s insulating envelope should be continuous and uniform. There should not be any points where heat can escape more easily than at others. However, in practice it is difficult to achieve a perfectly continuous insulating envelope. Breaks in this envelope are called thermal bridges.
Thermal bridges are inherent to many construction systems and linked to design flaws or sub-optimal building practices.
Avoiding thermal bridges
As we attempt to increase insulation and maximise a building’s energy performance, it becomes increasingly important to avoid thermal bridges. There is little point in increasing the thickness of the insulation if it is interrupted by thermal bridges. These thermal bridges are the weakest links in the insulating barrier.
Thermal bridges can also cause condensation problems that can damage the insulating layer, further reducing its effectiveness and leading to a vicious circle of damage and reduced energy efficiency.
Breaking thermal bridges
The most classic example of a thermal bridge is the gap in insulation where a floor meets an outside wall.
In this case, the most effective solution is to insulate the outside surface of the walls, so the insulation covers the façade in a continuous envelope that is independent of the number of floors the building has.
The same problem often occurs at junctions between walls and the roof and between walls and the floor. To be effective, wall insulation must touch the roof and floor insulation so that no heat conducting elements traverse the entire thickness of the insulating layer.
Aluminium windows are another example. Aluminium is an excellent heat conductor, so there is little point in installing high-performance double-glazing in an aluminium frame if steps are not taken to break the thermal bridge formed by the window frame.
What is the answer?
It is not always easy to avoid thermal bridges. How serious they are depends on the desired level of energy performance. Solutions should first be sought at the design stage and in the choice of construction system. Then, it is a question of following best practices during the building phase.
The increased cost of energy, the threat of climate change and the need to reduce running costs are all arguments in favour of ensuring a building is well insulated. What is more, a well-insulated building will age better, require less maintenance, use less energy and therefore cost less to run. In France, as in many countries, installing effective insulation is also a legal requirement.
R.T 2005 an 2012
Buildings are undoubtedly the largest single users of energy in France, accounting for more than 40% of the country’s energy consumption and 20% of its CO2 emissions. In order to improve this situation, strict building energy regulations were introduced in 2005 (RT 2005) with the aim of reducing the energy consumed by new buildings by at least 15%. This percentage was to be increased every five years to attain a reduction of 40% by 2020.
New buildings must now meet the standards contained in an updated set of regulations, introduced following the Grenelle Energy Summit (RT 2012). These regulations were designed to favour the development of low-energy buildings, reduce heat losses through walls and windows, and reduce losses via thermal bridges by 20%. They also encourage the use of renewable energies (e.g., solar), sustainable heating systems (e.g., heat pumps, low-temperature boilers) and bioclimatic design (e.g., building orientation). RT 2012 standards have applied to tertiary buildings since 1st July 2011 and to residential buildings since 1st January 2013. Energy performance limits vary from region to region, depending on the local climate, which is taken into account when defining a building’s energy needs. They also depend on the building’s purpose, as some buildings, such as factories, require more energy than others in order to power machinery. All new buildings can meet the requirements set out in these regulations by combining good insulation with good building practices. Limits for a building’s energy requirements take into account the production of hot water, heating, cooling (e.g., air-conditioning systems), ancillary electrical equipment (e.g., ventilation systems) and lighting.
Overcoming the cold-wall effect
Perceived temperature is the mean between the temperature of the wall and the temperature in a room. If a building is poorly insulated, the walls will be cold, which will reduce the perceived temperature. This is known as the cold-wall effect.
The best way to eliminate this effect is to ensure walls are properly insulated. In addition to reducing heat losses, effective wall insulation will allow the room to be heated to a lower temperature.
In concrete terms, for a house to have a perceived temperature of 18°C, it will have to be heated to 22°C if the walls are poorly insulated but to only 19°C if the walls are well insulated.
In addition, effective wall insulation will help ensure heat is distributed much more uniformly throughout the room.
What is more, reducing your thermostat by 1°C, will reduce your heating bills by 7%.
Crédits: Guide de l’Isolation OOreka